1. Field of the Invention
The present invention relates to an apparatus and a method for inspecting the appearance of electronic parts. In particular, the present invention relates to an apparatus and a method for inspecting the appearance of electronic parts, which are preferably used to inspect any appearance defect of a measurement objective which equivalently constitutes a capacitor.
2. Description of the Related Art
A piezoelectric/electrostrictive element has been hitherto known, for example, in the fields of optics and precision manufacturing, as a displacement element capable of adjusting the optical path length or the position on the order of submicron.
A representative structure of the piezoelectric/electrostrictive element is shown in FIG. 10. The piezoelectric/electrostrictive element 100 comprises a substrate 102 formed of ceramic or the like, and a piezoelectric/electrostrictive operating section 104 formed on a first principal surface of the substrate 102.
The substrate 102 has at least one hollow space 106 on the inside. A thin-walled section of the substrate 102, which is formed to cover the hollow space 106, functions as a vibrating section 108. A thick-walled section of the substrate 102, other than the vibrating section 108, functions as a fixed section 110 for supporting the vibrating section 108.
The piezoelectric/electrostrictive operating section 104 is formed in an integrated manner by successively stacking, on the first principal surface of the substrate 102, a thin film-shaped lower electrode 112b, a piezoelectric/electrostrictive layer 114, and an upper electrode 112a. This is followed by sintering. When the piezoelectric/electrostrictive operating section 104 is recognized as an equivalent circuit, it constitutes a capacitor.
A brief explanation will now be given of the principle of operation of the piezoelectric/electrostrictive element 100. Intially, the piezoelectric/electrostrictive element 100 is subjected to a polarization treatment for the piezoelectric/electrostrictive layer 114 by applying, for a predetermined period of time, a polarization voltage which is higher than a driving voltage between the upper electrode 112a and the lower electrode 112b. After that, the driving voltage is applied between the upper electrode 112a and the lower electrode 112b so that the electric field-induced strain is generated. As a result, as shown in FIG. 11, the displacement occurs, for example, in a first direction (direction for the upper electrode 112a to face the free space).
As shown in FIG. 12, minute bubbles 116 exist in the piezoelectric/electrostrictive layer 114 in some cases. When a relatively high voltage is applied for a certain period of time in order to perform a polarization treatment or a withstand voltage test, it is feared that any breakdown that occurs is a result of the bubbles 116. If the breakdown occurs, as shown in FIGS. 13A and 13B, the upper electrode 112a is broken simultaneously with the piezoelectric/electrostrictive layer 114. If the degree of breakage (hereinafter referred to as "breakage ratio" (breakage area/electrode area)) is larger than a predetermined value (appearance reference value), the function of the piezoelectric/electrostrictive element 100 is damaged. Therefore, it is necessary to exclude any products appearing to have bubbles.
In other cases, as shown in FIG. 12, the metal (for example, Au), which constitutes the upper electrode 112a and the lower electrode 112b, permeates the piezoelectric/electrostrictive layer 114 of the piezoelectric/electrostrictive element 100. If the insulation performance of the piezoelectric/electrostrictive layer 114 is damaged by the permeation of the metal, it is impossible to have the piezoelectric/electrostrictive element 100 to function normally. In these cases, it is necessary to exclude any products having an insulation defect.
In order to inspect those having the defects described above, a visual observation has been hitherto used to determine the appearance defect, and insulation resistance has been hitherto used to test for the insulation defect. In the appearance inspection method based on the visual observation, an inspection operator inspects the appearance defect by observing the surface of the upper electrode 112a of the piezoelectric/electrostrictive element 100 by using a microscope or the like. If the upper electrode 112a is broken, the presence or absence of the appearance defect of the piezoelectric/electrostrictive element 100 is judged by comparing the breakage ratio of the upper electrode 112a with an appearance reference value by means of visual observation.
On the other hand, the insulation inspection method based on the detection of the insulation resistance is performed as follows. A predetermined voltage (for example, 1 V) is applied between the upper electrode 112a and the lower electrode 112b of the piezoelectric/electrostrictive element 100. In this state, the value of a current flowing between the both electrodes 112a, 112b is detected by using an ammeter. Thus, the insulation defect of the piezoelectric/electrostrictive element 100 is inspected. In this method, the insulation defect of the piezoelectric/electrostrictive element 100 is judged depending on the presence or absence of the current. That is, it is determined that the insulation defect exists if the detected current value is larger than zero. If the detected current value is zero, it is determined that no insulation defect exists.
However, in the case of the appearance inspection method based on the visual observation, the inspection operator has to inspect each piezoelectric/electrostrictive element 100 individually. Therefore, a limit on examination is imposed in order to shorten the inspection time. Moreover, there is a possibility that the judgement is made on the basis of a subjective standard of the inspection operator. For this reason, it is feared that the accuracy of the inspection is lowered.
The following appearance inspection methods may be conceived in order to solve the problems as described above. The first method is a method for inspecting the defect of the piezoelectric/electrostrictive element 100 by detecting the capacitance value of the piezoelectric/electrostrictive element 100 by using an ammeter (hereinafter referred to as "ammeter-based appearance inspection method"). The second method is a method for inspecting the defect of the piezoelectric/electrostrictive element 100 by detecting the breakdown sound associated with the breakdown causing the appearance defect (hereinafter referred to as "sound-based appearance inspection method").
The ammeter-based appearance inspection method is performed as follows to detect the capacitance value. At first, the upper electrode 112a and the lower electrode 112b are charged with saturated electric charge therebetween. A value of a current which flows when the electric charge is discharged is detected by using the ammeter. If the upper electrode 112a of the piezoelectric/electrostrictive element 100 is broken, the capacitance value is lowered depending on the breakage ratio. Accordingly, the detected capacitance value is compared with a capacitance value (appearance reference capacitance value) corresponding to a preset appearance reference value. Thus, the piezoelectric/electrostrictive element 100 is inspected for the presence or absence of the appearance detect.
The ammeter-based appearance inspection method is advantageous in that the defect in appearance can be detected objectively.
On the other hand, the sound-based appearance inspection method is carried out as follows. A polarization treatment or a withstand voltage test is performed for the piezoelectric/electrostrictive element 100. The piezoelectric/electrostrictive element 100 is inspected for the appearance defect on the basis of the sound pressure of the breakdown sound or the sound pressure at a specified frequency (power spectrum) brought about in accordance with the breakdown of the piezoelectric/electrostrictive layer 114, which is caused during this process.
When the breakdown sound is detected, the presence or absence of the appearance defect is judged by comparing the generated breakdown sound with a sound pressure value (appearance reference sound pressure value) previously set on the basis of the appearance reference value.
The sound-based appearance inspection method is also advantageous in that the appearance of the piezoelectric/electrostrictive element can be objectively inspected in the same manner as in the ammeter-based appearance inspection method.
However, many problems remain to be solved in order to practically use either the ammeter-based appearance inspection method and the sound-based appearance inspection method.
In the ammeter-based appearance inspection method, the appearance defect is inspected by comparing the capacitance value of the piezoelectric/electrostrictive element 100 with the appearance reference capacitance value. However, the appearance reference capacitance value does not correspond to the appearance reference value in some cases. That is, the appearance defect occasionally exists even when the capacitance value of the piezoelectric/electrostrictive element 100 satisfies the appearance reference capacitance value.
Such a situation occurs due to the self-repairing function of the capacitor. The self-repairing function is based on a phenomenon caused when the electrode of the capacitor is broken. In such a case, the broken cross section of the electrode functions as a new electrode surface, and the ratio of decrease in capacitance value becomes smaller than those assumed from an actual breakage ratio of the electrode area. Therefore, when the appearance defect is inspected by means of the ammeter-based appearance inspection method, there is a possibility that any piezoelectric/electrostrictive element 100 having the appearance defect is erroneously judged to be an adequate product, and the appearance defect is missed. If the appearance reference capacitance value is strictly set in order to avoid the misidentification of the appearance defect, usable piezoelectric/electrostrictive elements 100 are excluded, resulting in an uneconomical identification.
The change in capacitance value, which results from the appearance defect of the piezoelectric/electrostrictive element 100, is minute. Therefore, the change in electric charge, which is charged between the upper electrode 112a and the lower electrode 112b, is also minute. In order to detect the change in electric charge, it is necessary to use an ammeter having extremely high performance. However, it is difficult to construct such an ammeter at present. Further, even if such an ammeter can be constructed, it is assumed that it would be extremely expensive.
In the ammeter-based appearance inspection method described above, the current must be detected directly for the individual piezoelectric/electrostrictive element 100. Therefore, both the inspection apparatus and the inspection steps become necessarily complicated. Further, it is difficult to apply the ammeter-based appearance inspection method to any integrated piezoelectric/electrostrictive element 100.
Therefore, it is difficult that the ammeter-based appearance inspection method is put into practice at present in view of the accuracy, technology, economics, and efficiency.
On the other hand, in the sound-based appearance inspection method, the appearance defect is inspected on the basis of the detected sound pressure. Therefore, this method is greatly affected by noise due to its character. Accordingly, in order to successfully detect the minute change in sound pressure, it is necessary to use an inspection environment which is extremely excellent in sound insulation performance and sound isolation performance. However, it is technically difficult to realize such an inspection environment. Further, even if such an inspection environment can be technically realized, it is assumed that it would be extremely expensive.
Therefore, it is also difficult that the sound-based appearance inspection method is put into practice at present in view of the accuracy, technology, and economics.